Combined luminescent screen and antidiffusion grid and method of making same

ABSTRACT

A combined X-ray luminescent screen and antidiffusion grid, in which compartmenting of the phosphor layer is partly or wholly achieved by means of the same X-ray absorbing and lightreflecting strips which also form the antidiffusion grid portion of the assembly. The interspaces between such strips are partly filled with the phosphor and partly are left empty or filled with a material transparent for X-rays.

[ 1 Feb. 15, 1972 United States Patent van der Feyst [56] References Cited UNITED STATES PATENTS [54] COMBINED LUMINESCENT SCREEN AND ANTIDIFFUSION GRID, AND METHOD OF MAKING SAME .250/71 R X ....250/80 X ....313/92 LF Garrison 2,985,784 5/1961 MacNeille. 3,041,228 MacLeod...

Delft, Netherlands Mar. 2, 1970 Primary Examiner-Archie R. Borchelt Attorney-Arthur B. Colvin [22] Filed:

Appl. No.2 15,820

[57] ABSTRACT A combined X-ray luminescent screen and antidifinsion grid,

[30] Foreign Application Priority Data in which compartmenting of the phosphor layer is partly or wholly achieved by means of the same X-ray absorbing and Mar. 5, 1969 Netherlands..........................6903366 light-reflecting strips which also form the antidiffusion grid [22] 250/77, 313/92 LF portion of the assembly The interspaces between such Strips 1 n are partly filled with the phosphor and partly are left empty or g filled with a material transparent for X-rays.

5s FieldofSearch............

7 Claims, 8 Drawing Figures PATENTEIJFEB 1 5 I912 VIII!!!Ilfiflfllflllllllf'llll:

lllluuuuuuuu mam INVENTOR.

BYM/ J COMBINED LUMINESCENT SCREEN AND ANTIDIFFUSION GRID, AND METHOD OF MAKING SAME The invention relates to X-ray luminescent screens and, more particularly, to screens of the compartmented type. In such screens the phosphor layer is usually subdivided into a large number of small dots which are optically separated from each other by thin walls, usually metallic, which are strongly light reflective and extend substantially parallel to the primary X-rays.

The purpose of such compartmenting is to confine light generated in a phosphor particle and propagated as stray light in all directions, to the space enclosed by the adjacent light reflecting walls. Compared to screens with a continuous phosphor layer, the compartmentedscreens may thus be made thicker and, with a given resolving power or definition, may have a better energy conversion efficiency.

The series production of screens regularly divided in thousands of compartments having dimensions in the order of some tenths of a millimeter constitutes a problem which hitherto could not be solved satisfactorily. For instance, according to one method as described in the US. Pat. No. 3,041,456, a rectangular body of plastic in which the phosphor has been dispersed, is sliced into thin slices which are coated on one or both sides with a reflective material. Then the body is reformed by bonding the slices back together and again sliced along planes transverse to the first slices. The coating and bonding operations are then repeated to obtain a doubly laminated body from which screens of the desired thickness may finally be obtained by slicing along planes normal to both laminations.

It is an object of the invention to provide a compartmented X-ray luminescent screen which can be fabricated with relatively simple means and in accordance with a less cumbersome method. It is a further object to provide a screen which, due to its structure and the materials applied, also inherently functions as an X-ray antidiffusion grid.

To achieve these and other objects the compartmented screens according to the invention, in the broadest aspect thereof, is characterized in that at least a part of the separating walls are formed by evenly spaced apart substantially flat strips which, in addition to being highly light reflective, consist of a material having relatively high absorption for X-rays, the spaces between adjacent strips containing luminescent material along part of their height only, and along the remainder of their height which is a multiple of the spacing distance between adjacent strips, either being empty or containing a material which has relatively low absorption for X-rays.

In use, the phosphor side of the screen is turned toward the observer, or to the lens of an X-ray fluorographic camera, as the case may be. The other side is thus facing the X-ray source. Due to their X-ray absorbing properties, the narrowly spaced strips will only give passage to those rays which are directed substantially along the primary X-ray beam. Diffused X-rays, such as secundary radiation coming from the patient under examination, are stopped by the strips and are thus prevented from spoiling the delicate contrasts of the primary X-ray image formed in the screen. As is well known, it is common practice to use separate antidiffusion grids or Buckygrids for this purpose. Conventional Bucky-grids are flat panels placed before the X-ray screen and made up of thin heavy-metal strips spaced apart by spacer strips of nonabsorbing material, such as pertinax. Another well-known type of antidiffusion grids has a circular form and is made by winding a thin heavy-metal strip together with a spacer strip in the fashion of a spiral. Antidiffusion or Bucky-grids are presently being manufactured in large quantities in accordance with well-mastered production methods.

It will be understood that the combined luminescent screen and antidiffusion grid according'to the invention, in its simplest form, may be made in a manner similar to a conventional antidiffusion grid, the spacers, however, being given a height somewhat less than that of the absorbing strips and the empty spaces thus left between the absorbing strips being filled with the phosphor afterwards. The double-function panel thus obtained, compared to a continuous screen with separate antidiffusion grid, has the advantage of being compartmented in one direction, which may sufficiently improve the performance of the screen for certain purposes.

Preferably, however, the screen according to the invention will be compartmented in two directions in that between adjacent flat strips highly light-reflective waved or corrugated strips are inserted acting as spacers between the flat strips.

Clearly, if such corrugated strips have the same height as the plane strips and are made of an X-ray absorbing material, e.g., the same material as the flat strips. then the antidiffusion properties of the resulting assembly will be similar to those of a pair of crossed Bucky-grids. If, however, the corrugated strips have a much smaller height, more particularly one that does not exceed the thickness of the phosphor layer envisaged, then the screen will act upon stray X-rays in much the same way as a single conventional Bucky-grid.

Experience has shown that the production of fully compartmented grids using corrugated strips as spacers is not essentially more complicated or difficult than with the conventional flat spacers. Thus, to obtain a rectangular screen, flat strips and corrugated strips having the length of the screen may be alternatingly placed against each other until a panel of the desired breadth has been formed. For a circular screen a long flat strip and a corrugated strip may be wound together in a spiral fashion until a disc of the desired diameter is obtained.

' Then, in accordance with a preferred method of making screens according to the invention, the panel or disc thus obtained is placed with one side in an easily impressible layer of a solvable matter with a thickness equal to that of the phosphor layer to be applied. Next the interspaces not filled by the solvable matter are filled with a self-hardening liquid. Finally,

'after solidification of the liquid, the solvable matter is removed with a suitable solvent and the phosphor is brought into the compartments.

It should be observed that it is well-known per se to form a large number of narrow channels in an image forming device by stacking alternatingly flat and corrugated strips or sheets (compare for instance French patent of addition 82.267 and British Pat. No. 1.064.075). These prior art applications, however, do not concern luminescent screens but so-called multichannel image intensifier devices.

It should further be observed that it is also known from the Dutch Pat. No. 100.12l, in an X-ray screen having separate recessions containing the phosphor, to make the walls of such recessions of a material which is not only light reflective but in addition X-ray absorbing. The purpose thereof is mainly to prevent secundary X-rays emanating from the phosphor itself from crossing over to the next-adjacent recessions and giving rise to unwanted light therein. Evidently therefore, these walls can not be compared to the conventional antidiffusion grids discussed hereinbefore. It is true that the Dutch patent specification also suggests to fill the separate recessions only partly with phosphor so that the walls extend somewhat beyond the phosphor. However, they do so in the direction toward the observer and not, as do the walls in the screen according to the present invention, toward the X-ray source.

Details of the invention will now be described with reference to the accompanying drawing which shows in:

FIG. 1 a perspective view, partly in cross section, of a small portion of a first screen according to the invention, using solid strips as spacers between the X-ray absorbing strips;

FIG. 2 a top view of a grid for a second type of screen according to the invention, during its formation this screen being circular and using a corrugated strip to provide the required spacing between the X-ray absorbing strips and to form compartments;

FIG. 3 a similar top view as FIG. 2 of a rectangular grid during its formation;

FIG. 4 a cross-sectional view of a portion of a completed screen using grids according to FIG. 2 or 3;

FIG. 5 a perspective view of the strip configuration as used in the screen of FIG. 4;

FIG. 6 a similar perspective view ofa different strip configuration;

FIG. 7 a cross-sectional view of an arrangement which may be used for producing screens having their X-ray absorbing strips directed to the X-ray focus; and

FIG. 8 a cross-sectional view ofa screen produced by means of the arrangement of FIG. 7.

The screen partly shown in FIG. 1 is made up of thin metallic strips I spaced apart by solid strips 2 ofa material which is transparent for X-rays in a manner similar to conventional antidiffusion grids. However, the strips 2 are of somewhat less height than the strips 1 and, in addition, the material of the latter strips is so selected that they not only absorb the secondary X-rays incident thereon but also have a highly lightreflective surface.

Thus, on the side remote from the X-ray source, narrow grooves having reflective sidewalls are formed which are filled with the phosphor 3. Preferably, the material of strips 2 is also light reflective, e.g., white, in order to have as much of the light emanating from the phosphor emitted from the screen.

It will be appreciated that the walls 1 confine the luminescent light to the groove in which it is formed and this results in a markedly improved definition of the screen in the direction transverse to the strips 1, compared to a continuous screen of equal conversion efficiency. In certain cases such a one-directional compartmenting of the screen may be desirable therefore.

Where full advantage should be taken from compartmenting the screen, recourse may be had to the embodiments illustrated in FIGS. 2 to 6, which make use of corrugated strips as a means of spacing apart the flat strips. As seen in FIG. 2, a circular screen with a fully compartmented phosphor layer can be obtained by winding together in a spiral a long flat strip 4 and a corrugated or waved strip 5 until a disc 6 of the desired diameter has been formed. This disc contains a large number of compartments 7 having substantially equal dimensions and shape. Into these compartments the luminescent material can be brought. As seen in FIG. 3, a compartmented rectangular screen 10 can be made by placing alternatingly against each other flat strips 8 and corrugated strips 9, each having the length of the screen, until the desired breadth has been reached.

The cross-sectional view of FIG. 4 shows on a larger scale that the interspaces 12 between the flat strips 11 and the corrugated strips 16 are filled with the phosphor 13 along a small portion of their height only. Along the remaining height they are filled with a solid material 14 giving the required mechanical strength to the screen. This material should have a good transparency to X-rays in order to attenuate the primary X- rays as little as possible. To protect the strip edges the upper side of the screen is covered with a thin layer of the same material.

The ratio of the height of the spaces filled with the material 14 and the mutual distance of the strips 11 determines the effi' ciency of the assembly as an antidiffusion grid. In practice this ratio, as with the conventional Bucky-grids, can be varied between wide limits, but preferably it is made to exceed three.

In the configuration shown in FIG. 5, like in FIG. 4, the corrugated strips I6 have the full height of the screen, thus defining prismatic compartments 12 of which only the lower part will be filled with the phosphor. In FIG. 6, on the contrary, the corrugated strips 16 are much lower than the flat strips 11. Only the interspaces 12" will be filled with the phosphor, whereas the remaining spaces 12' between the flat strips 11 will be left empty or will be filled with a supporting material transparent for X-rays. The latter form may be compared in effect to a single Bucky-grid.

The metal strips to be used should of course absorb the secundary X-rays as completely as possible, at least as far as the flat strips are concerned. However, it is also desirable to use strip which is as thin as possible in order to keep the useful area of the screen large. It is desirable therefore to apply metals of high atomic weight which thus have a high specific absorption for X-rays. Gold and silver in a thickness of 0.03 to 0.04 mm. for instance have shown to be suitable, also because of their high reflection for light and their suitability to be transformed to a corrugated or waved strip. It will be clear, though, that the objects of the present invention will be achieved already to a high degree if, in the embodiments illustrated in FIGS. 4 and 5, only the flat strips are made of such a heavy metal, and the other is made e.g., of a lighter metal. Also, the strips may consist of a suitable alloy of light and heavy metals or may have a stratified combination of such different materials.

The transverse dimensions of the compartments depend mainly on the desired resolving power of the screen. For many applications a distance between the flat strips of 0.3 to 1 mm. appears to be sufficiently small to avoid any disturbing influence of the grid structure on the X-ray image. The most favorable thickness of the phosphor layer depends on the penetrating power of the primary X-rays and on the light diffusing and light absorbing properties of the phosphor. Preferably, the thickness at which optimal light conversion efficiency is achieved, is determined by some experimentation. It appeared, for instance, that in the case ofa certain medical X-ray image intensifier camera a thickness of 0.6 to 0.7 mm. was best suitable. In either case, the phosphor thickness is of no influence, in a fully compartmented screen, on the resolving power ofthe screen, as the latter is completely determined by the transverse dimensions of the compartments.

With reference to FIG. 7 a preferred method for manufacturing a screen according to the invention will now be described. The screen concerned is a so-called focused screen whose separating walls are directed to the focus of the X-ray tube to be used in conjunction with the screen. The use of focused screens is desirable in order that the separating walls cause as little shadowing in the X-ray image as possible.

A disc or panel 17, which may have been prepared in the manner illustrated in FIGS. 2 or 3, is placed on the convex upper surface ofa table 18. This surface may be spherical for a circular screen, or cylindrical for a rectangular screen. To the surface a layer 19 of an easily impressible and solvable matter has been applied. Mixtures of paraffin wax and Vaseline ap pear to be suitable for this. Layer I9 has the thickness of the phosphor layer envisaged. The originally flat disc or panel 17 is carefully pressed until it bears on the table 18 throughout its surface. During this step adjacent windings or strips of the grid will slide somewhat alongside each other but they remain strictly vertical.

Thereafter the remaining spaces over the layer 19 are filled with a suitable self-hardening liquid. For example, thermosetting plastics, such as an epoxy resin known under the trade name Araldite, may be used therefor. Since this is an optically clear liquid itself a white pigment is added thereto in order that light which is radiated in the direction of the X-ray source, be reflected for the greater part and added to the useful light. With a little excess of plastic and by applying a frame 21 and a counter mould 22 the screen may also be provided with a protecting cover 23 and a reinforcing edge 20.

Before the self-hardening liquid becomes completely solid screen 17 is taken up from table 18 and disposed upside down in a trough containing a suitable solvent for the material 19, such as petrol. At the same time, however, the screen is pressed to flatness again, as seen in FIG. 8. Thereby a slight deformation of the screen windings or strips occurs which produces the focused position of the strips. In this condition the plastic can be left to finish setting.

Matter 19 in the meantime is washed out of the compartments. These are now to be filled with the phosphor. This can conveniently be done by strawing the phosphor into the same solvent so that it will be sedimented in the compartments. Finally, the screen can be taken out and dried, after a possible excess of phosphor has been removed. To prevent dropout of phosphor particles the relevant side of the screen can be fixed e.g., by means ofa thin layer 15 of polyvinyl alcohol (see FIG. 3).

It may be observed that of course other means may be conceived to achieve focusing of the separating walls. For instance, if winding a disc in the manner illustrated in FIG. 2, the outer strip may be constantly pressed during winding onto the foregoing strip by means of a roller which is supported by a rod journaled in a point in the optical axis of the screen, at a distance equal to the focal distance. I

What I claim is:

1. X-ray luminescent screen comprising a large number of compartments containing luminescent material which are separated from one another by thin light-reflective walls extending substantially parallel to the primary X-rays, characterized in that at least part of the separating walls are formed by evenly spaced apart substantially flat strips which consist of a material having relatively high absorption for X-rays, the spaces between adjacent strips containing luminescent material along part of their height only, and along the remainder of their height, which is a multitude of the spacing distance between adjacent strips, either being empty or containing a material which has relatively low absorption for X-rays.

2. X-ray luminescent screen as claimed in claim 1 wherein between said flat strips light-reflective corrugated strips are inserted which act as spacers between such flat strips and define additional walls for said compartments.

3. X-ray luminescent screen as claimed in claim 2 wherein said corrugated strips have the same height as said flat strips and consist likewise of a material having relatively high absorption for X-rays.

4. X-ray luminescent screen as claimed in claim 2, wherein said corrugated strips have a height substantially equal to the thickness of the luminescent layer.

5. X-ray luminescent screen as claimed in claim 1 wherein the remainder of the spaces between said flat strips contain a material which is light reflecting.

6. Method of making an X-ray luminescent screen, comprising the steps of: forming a grid of substantially flat high X-ray absorbing strips spaced by light reflecting corrugated strips, placing said grid with one side in an easily impressible layer of a solvable matter having a thickness equal to that of the luminescent layer to be formed, filling the remaining interspaces over said solvable matter with a self-hardening liquid. removing said solvable matter after solidification of said liquid. and bringing luminescent material into the compartments vacated by said solvable matter.

7. Method as claimed in claim 6, of making a focused screen wherein said easily impressible layer is applied to a convex surface against which said grid is pressed, and wherein said grid is pressed to flatness again before solidification of said self-hardening liquid is completed. 

2. X-ray luminescent screen as claimed in claim 1 wherein between said flat strips light-reflective corrugated strips are inserted which act as spacers between such flat strips and define additional walls for said compartments.
 3. X-ray luminescent screen as claimed in claim 2 wherein said corrugated strips have the same height as said flat strips and consist likewise of a material having relatively high absorption for X-rays.
 4. X-ray luminescent screen as claimed in claim 2, wherein said corrugated strips have a height substantially equal to the thickness of the luminescent layer.
 5. X-ray luminescent screEn as claimed in claim 1 wherein the remainder of the spaces between said flat strips contain a material which is light reflecting.
 6. Method of making an X-ray luminescent screen, comprising the steps of: forming a grid of substantially flat high X-ray absorbing strips spaced by light reflecting corrugated strips, placing said grid with one side in an easily impressible layer of a solvable matter having a thickness equal to that of the luminescent layer to be formed, filling the remaining interspaces over said solvable matter with a self-hardening liquid, removing said solvable matter after solidification of said liquid, and bringing luminescent material into the compartments vacated by said solvable matter.
 7. Method as claimed in claim 6, of making a focused screen wherein said easily impressible layer is applied to a convex surface against which said grid is pressed, and wherein said grid is pressed to flatness again before solidification of said self-hardening liquid is completed. 